U.S. patent number 6,631,679 [Application Number 10/071,541] was granted by the patent office on 2003-10-14 for printing plate material with electrocoated layer.
This patent grant is currently assigned to Alcoa Inc.. Invention is credited to David S. Bennett, Sallie L. Blake, Robert E. Bombalski, Kenneth A. Bowman, Joseph D. Guthrie, Thomas L. Levendusky, Daniel L. Serafin, Jean Ann Skiles.
United States Patent |
6,631,679 |
Bennett , et al. |
October 14, 2003 |
Printing plate material with electrocoated layer
Abstract
A process for making printing plate material suitable for
imaging by laser radiation. A metal substrate is electrocoated in a
bath containing a polymeric resin and laser-sensitive particles,
thereby depositing a laser ablatable layer on a principal surface
of the metal substrate. In one embodiment, the laser-ablatable
layer is treated with a corona discharge for a time sufficient to
render the layer non-ink wettable. In other preferred embodiments,
the laser-ablatable layer is overcoated with an overlayer such as a
non-ink wettable silicone layer or a water-wettable layer
comprising an organophosphorus polymer, preferably a copolymer of
acrylic acid and vinylphosphonic acid.
Inventors: |
Bennett; David S. (Davenport,
IA), Blake; Sallie L. (Long Grove, IA), Bombalski; Robert
E. (New Kensington, PA), Bowman; Kenneth A. (Apollo,
PA), Guthrie; Joseph D. (Murrysville, PA), Levendusky;
Thomas L. (Greensburg, PA), Serafin; Daniel L. (Wexford,
PA), Skiles; Jean Ann (Gibsonia, PA) |
Assignee: |
Alcoa Inc. (Pittsburgh,
PA)
|
Family
ID: |
27059679 |
Appl.
No.: |
10/071,541 |
Filed: |
February 8, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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644010 |
Aug 22, 2000 |
6374737 |
|
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519018 |
Mar 3, 2000 |
6405651 |
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Current U.S.
Class: |
101/457;
101/459 |
Current CPC
Class: |
B41C
1/1033 (20130101) |
Current International
Class: |
B41C
1/10 (20060101); B41N 001/08 () |
Field of
Search: |
;101/454,457,458,459,463.1,465,466,467 ;204/492-498
;205/139,153,213,214,317 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Funk; Stephen R.
Attorney, Agent or Firm: Meder; Julie W. Klepac; Glenn
E.
Parent Case Text
RELATED APPLICATION
This application is a divisional application of U.S. Ser. No.
09/644,010 filed Aug. 22, 2000 entitled "Printing Plate Material
With Electrocoated Layer", now U.S. Pat. No. 6,374,737 which is a
continuation-in-part application of U.S. Ser. No. 09/519,018 filed
Mar. 3, 2000 entitled "Electrocoating Process for Making
Lithographic Sheet Material", now U.S. Pat. No. 6,405,651.
Claims
What is claimed is:
1. A printing plate comprising: a metal substrate having a
principal surface; and a laser-ablatable layer electrocoated onto
said principal surface, wherein said principal surface is roll
textured, mechanically textured, chemically textured and/or
electrochemically textured, and wherein said laser-ablatable layer
comprises an oleophilic material and has a water wettable upper
surface.
2. The printing plate of claim 1 wherein said principal surface is
roll textured.
3. The printing plate of claim 1 wherein said principal surface is
roll textured, said principal surface having an extended surface
area of about 0.05-10%.
4. The printing plate of claim 3 wherein said principal surface has
a surface roughness of about 5 to less than 15 microinches.
5. The printing plate of claim 3 wherein said principal surface has
a surface roughness of about 6 to 9 microinches.
6. The printing plate of claim 1 wherein said metal substrate
comprises aluminum or an aluminum alloy.
7. The printing plate of claim 6 wherein said laser ablatable layer
is an anodic electrocoated layer.
8. The printing plate of claim 7 wherein said principal surface
comprises a layer of a nonporous anodic oxide of said metal.
9. The printing plate of claim 7 wherein said laser ablatable layer
is a cathodic electrocoated layer.
10. The printing plate of claim 9 wherein said principal surface
comprises a pretreatment layer, said pretreatment layer comprising
a conversion coating or an electrochemically deposited coating.
11. The printing plate of claim 10 wherein said conversion coating
comprises a salt of Zn, Cr, P, Zr, Ti or Mo.
12. The printing plate of claim 10 wherein said electrochemically
deposited coating is an anodic oxide.
Description
FIELD OF THE INVENTION
The present invention relates to printing plate materials suitable
for imaging by digitally controlled laser radiation. More
particularly, the invention relates to printing plate materials
having an electrocoated layer thereon.
BACKGROUND OF THE INVENTION
Printing plates suitable for imaging by digitally controlled laser
radiation are produced commercially. However, the existing
processes for making such plates are expensive and wasteful.
Accordingly, there still remains a need for a more efficient and
economical process of making such plates.
Laser radiation suitable for imaging printing plates preferably has
a wavelength in the near-infrared region, between about 400 and
1500 nm. Solid state laser sources (commonly termed "semiconductor
lasers") are economical and convenient sources that may be used
with a variety of imaging devices. Other laser sources such as
CO.sub.2 lasers and lasers emitting light in the visible
wavelengths are also useful.
Laser output can be provided directly to the plate surface via
lenses or other beam-guiding components, or transmitted to the
surface of a blank printing plate from a remotely sited laser
through a fiber-optic cable. A controller and associated
positioning hardware maintains the beam output at a precise
orientation with respect to the plate surface, scans the output
over the surface, and activates the laser at positions adjacent
selected points or areas of the plate. The controller responds to
incoming image signals corresponding to the original figure or
document being copied onto the plate to produce a precise negative
or positive image of that original. The image signals are stored as
a bitmap data file on the computer. Such files may be generated by
a raster image processor (RIP) or other suitable means. For
example, a RIP can accept data in page-description language, which
defines all of the features required to be transferred onto a
printing plate, or as a combination of page-description language
and one or more image data files. The bitmaps are constructed to
define the hue of the color as well as screen frequencies and
angles.
The imaging apparatus can operate on its own, functioning solely as
a platemaker, or can be incorporated directly into a lithographic
printing press. In the latter case, printing may commence
immediately after application of the image to a blank plate,
thereby reducing press set-up time considerably. The imaging
apparatus can be configured as a flatbed recorder or as a drum
recorder, with the lithographic plate blank mounted to the interior
or exterior cylindrical surface of the drum. Obviously, the
exterior drum design is more appropriate to use in situ, on a
lithographic press, in which case the print cylinder itself
constitutes the drum component of the recorder or plotter.
In the drum configuration, the requisite relative motion between
the laser beam and the plate is achieved by rotating the drum (and
the plate mounted thereon) about its axis and moving the beam
parallel to the rotation axis, thereby scanning the plate
circumferentially so the image "grows" in the axial direction.
Alternatively, the beam can move parallel to the drum axis and,
after each pass across the plate, increment angularly so that the
image on the plate "grows" circumferentially. In both cases, after
a complete scan by the beam, an image corresponding (positively or
negatively) to the original document or picture will have been
applied to the surface of the plate.
In the flatbed configuration, the beam is drawn across either axis
of the plate, and is indexed along the other axis after each pass.
Of course, the requisite relative motion between the beam and the
plate may be produced by movement of the plate rather than (or in
addition to) movement of the beam.
Regardless of the manner in which the beam is scanned, it is
generally preferable (for reasons of speed) to employ a plurality
of lasers and guide their outputs to a single writing array. The
writing array is then indexed, after completion of each pass across
or along the plate, a distance determined by the number of beams
emanating from the array, and by the desired resolutions (i.e., the
number of image points per unit length.)
Some prior art patents disclosing printing plates suitable for
imaging by laser ablation are Lewis et al U.S. Pat. Nos. 5,339,727
and 5,353,705 and Nowak et al. U.S. Pat. No. Re. 35,512. The
disclosures of those patents are incorporated herein, to the extent
consistent with our invention.
Although these prior art printing plates perform adequately, they
are expensive to produce because the absorbing layer is vapor
deposited onto the oleophilic polyester layer. Adhesive bonding of
the polyester layer to a metal substrate also adds to the cost.
A principal objective of the present invention is to provide a
printing plate material wherein a laser-ablatable layer is
deposited on a substrate by electrocoating. The electrocoating
process of our invention coats metal substrates at greater speed
and with improved quality compared to prior art processes such as
laminating, adhesive bonding, extrusion coating, and roll
coating.
A related objective of our invention is to provide a process
suitable for making both positive and negative lithographic
plates.
Additional objectives and advantages of our invention will become
apparent to persons skilled in the art from the following
description of some preferred embodiments.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an
improved process for making printing plate material suitable for
imaging by laser radiation. The process of our invention is useful
for making negative printing plates and for making positive
printing plates.
The process of the invention makes printing plate material by
coating a substrate with one or more polymeric layers. The
substrate is a metal, preferably an aluminum alloy or steel. Some
suitable aluminum alloys include alloys of the AA 1000, 3000, and
5000 series. Suitable steel substrates include mild steel sheet and
stainless steel sheet.
An aluminum alloy substrate should have a thickness of about 1-30
mils, preferably about 5-20 mils, and more preferably about 8-20
mils. An unanodized aluminum alloy substrate having a thickness of
about 8.8 mils is utilized in a particularly preferred
embodiment.
The substrate may be mill finished or, more preferably, may be
further finished via roll texturing, chemical texturing, mechanical
texturing, electrochemical texturing or combinations thereof. Roll
texturing may be accomplished with a roll having an outer surface
roughened via electron discharge texturing (EDT), laser texturing,
electron beam texturing, mechanical texturing, chemical texturing,
electrochemical texturing or combinations thereof. Preferred
mechanical texturing techniques include shot peening and brush
graining. A preferred technique for roll texturing is EDT. In EDT,
a plurality of arc generating electrodes are spaced from the outer
surface of the roll and pulses of electron arcs are discharged
against the roll outer surface. The arcs provide a generally
uniform roll surface of peaks and valleys of desired dimensions.
The electrodes rotate and traverse across the roll outer surface.
The dimensions are controlled at least in part by the voltage level
and the current level of the arcs, the length of the arc pulses,
the length of time between arc pulses, and the electrode rotational
speed and traverse rate. Electron discharge texturing is disclosed
in U.S. Pat. Nos. 3,619,881 and 4,789,447, both being incorporated
herein by reference.
When textured rolls, for example rolls subjected to EDT, are used
to roll the substrate, the surface area of the substrate is
increased (extended) in a non-directional manner. A preferred level
of surface area extension of a nominally flat aluminum sheet (mill
finished) is preferably about 0.5 to 10%. The surface of roughness
(Ra) of aluminum sheet rolled with EDT treated rolls is preferably
about 5 to less than 15 microinches, more preferably about 6 to
about 9 microinches.
The resulting textured surface provides a more diffuse surface than
a mill finished surface with concomitant higher uniformity in the
surface. During laser ablation, non-uniform surface defects have
been associated with laser back reflections. The textured surface
of the product of the present invention minimizes laser back
reflections and improves the uniformity and efficiency of the laser
ablation process.
A principal surface of the metal surface is cleaned to remove
surface contaminants such as lubricant residues. Some suitable
chemical surface cleaners include alkaline and acid aqueous
solutions. Plasma radiation and laser radiation may also be
utilized. After the principal surface is cleaned, it is coated with
a laser-ablatable layer by electrocoating. By the term
laser-ablatable it is meant that the material or layer is subject
to absorption of infrared laser light causing ablation thereof.
The electrocoating process of our invention may be either anodic
electrocoating or cathodic electrocoating. The anodic process
involves immersing a continuous coil of aluminum alloy sheet into
an aqueous electrocoating bath. The sheet is grounded and an
electric current is passed between a cathode in the bath and the
sheet which functions as the anode. The bath contains an emulsified
polymeric resin and may also include laser-sensitive particles
combined with an acrylic resin. Total solids content of the bath is
generally about 5-20 wt. %. Electric current passing through the
bath electrolyzes water, generates hydronium ions at the sheet
surface. The hydronium ions react with amine groups on the
polymeric resin, liberating the acrylic polymer that precipitates
on the sheet surface. Similarly, amine groups on molecules of
acrylic resin combined with the laser sensitive particles are also
neutralized, thereby precipitating the particles along with the
polymeric resin as a laser-ablatable layer on the sheet surface.
When the metal substrate is formed from an aluminum alloy, the
electric current also generates a thin layer of anodic oxide
between the aluminum substrate and the laser-ablatable layer. Prior
to electrocoating the aluminum substrate, the substrate typically
bears on its exposed surfaces (including the principal surface) an
inherent non-uniform hydrated aluminum oxide layer. This inherent
aluminum oxide layer generally contains flaws that may have been
caused by thermomechanical processing of the substrate or
contamination introduced by such thermomechanical processing (e.g.
lubricants or coolants) or via other handling procedures. Upon
application of the electric current, the inherent oxide layer is
removed and a nonporous anodic oxide layer forms in its place
between the substrate and the polymer layer. The nonporous anodic
oxide layer is a continuous layer without the flaws typical of the
inherent oxide layer of the aluminum substrate and is typically
about 50 to about 100 Angstroms thick.
In the cathodic electrocoating process of the present invention,
the substrate functions as the cathode. The cathode (substrate) is
bathed with an alkaline resin solubilized in an acidic solution.
Upon application of an electric current from an anode (the tank
containing the bath or a separate anode), the resin is dehydrated
and deposits on the substrate. In order to create a uniform surface
on the sheet rendering the substrate receptive to the
electrocoating (comparable to the nonporous anodic oxide layer of
the anodic electrocoated sheet), the substrate may be chemically
pretreated with a conversion coating or electrochemically
pretreated in an anodizing process to produce an anodic oxide layer
thereon. The conversion coating may include salts of zinc,
chromium, phosphate, zirconium, titanium and molybdenum.
chrome-phosphate conversion coating is particularly preferred.
Other suitable conversion coatings may contain silicates or other
metals such as vanadium, niobium, tantalum, and hafnium.
The laser-sensitive particles preferably are particles of a metal,
mineral or carbon having an average particle size of about 7
microns or less. The metal particles may be copper, cobalt, nickel,
lead, cadmium, titanium, iron, bismuth, tungsten, tantalum,
silicon, chromium, aluminum or zinc, preferably iron, aluminum,
nickel, or zinc. The mineral particles may be oxides, borides,
carbides, sulfides, halides or nitrides of the metals identified
above or clay. Clay includes aluminum silicates and hydrated
silicates such as feldspar and kaolinate. Carbon may be used in the
form of carbon black, graphite, lamp black or other commercially
available carbonaceous particles. Combinations of particles having
different compositions are within the scope of our invention. Iron
oxide particles having an average size of less than 1 micron are
particularly preferred. When the laser-sensitive particles are
included in the coating bath, the amount of the laser-sensitive
particles in the coating bath may be as low as 1 ppm and as high as
50 wt. %, is preferably about 1-10 wt. % and is about 5 wt. % in a
particularly preferred embodiment.
The emulsified polymeric resin in the bath preferably comprises a
polymer of acrylic acid or methacrylic acid, or their analogs and
esters, alone or in mixtures and copolymers with an epoxy resin.
Carboxylic acid groups on the acrylic polymer are neutralized by a
base, preferably an organic amine.
The electrocoating process is self-limiting. As the coating
thickness increases, the electrical resistance of the electrocoated
layer also rises until current can no longer flow thereby limiting
the amount of coating deposited. Coating thickness is also limited
by the speed at which the metal sheet passes through the bath and
by the bath composition. The coating may have a thickness of about
0.01-1 mil. A coating having a thickness of about 0.05-0.3 mil is
particularly preferred. The electrocoated layer is more uniform
than layers deposited by other means such as roll coating and
provides a consistent thickness of the layer on each coated
substrate and from batch to batch. The edge-center-edge differences
associated with roll coating are avoided. The laser-sensitive
particles make up about 5 wt. % of the coating in a particularly
preferred embodiment.
The electrocoated laser-ablatable layer of polymeric resin and
laser-sensitive particles is cured by heating to a temperature of
about 100-300.degree. C. for a few seconds or less.
In a first embodiment of the printing plate of the present
invention, the electrocoated sheet is oleophilic (i.e. ink
wettable) and may be used directly as a printing plate for
applications in which an ink-wettable top surface is desired. The
electrocoated polymer layer may be laser-ablated to expose the
principal surface of the substrate except in the location of the
desired image area. The metal substrate may act hydrophilic (i.e.
water wettable) or oleophilic depending on the water affinity and
ink affinity properties of the layers thereon. In a case where the
electrocoated polymer layer is oleophilic, the metal substrate will
act hydrophilic. When a conventional printing fountain solution
containing ink and water is used with the laser-ablated sheet, the
ink adheres to the polymer layer in the image area while water
adheres to the metal substrate in the background (non-image) area.
Alternatively, the image area may be laser-ablated to render the
image area hydrophilic and retain the background area as oleophilic
so that water adheres to the image area and ink adheres to the
background.
In this embodiment, it is also possible to laser ablate only a
portion of the electrocoated polymer layer so as to not expose the
underlying substrate. The laser ablation process may alter the ink
affinity of the polymer such that the partially ablated areas of
the printing plate become hydrophilic while the non-ablated areas
remain oleophilic.
In a second embodiment of the inventive printing plate, an upper
portion of the laser-ablatable layer of the electrocoated polymer
is made hydrophilic by treating the surface of the electrocoated
polymer layer. In this manner, an upper portion of the layer of
electrocoated polymer is hydrophilic while a lower portion remains
oleophilic. Treatment of the upper portion of the laser-ablatable
layer of the electrocoated polymer may be accomplished via corona
discharge treatment or by including inorganic particles therein to
render the electrocoated polymer hydrophilic.
As used herein, the term "corona discharge" refers to a treatment
in which air or other gas is ionized in close proximity to the
coating surface. Ionization of the gas is initiated by passing a
high voltage current through an electrode in close proximity to the
surface, thereby causing oxidation and other changes on the coating
surface. Corona discharge is typically operated with a power source
providing about 6-20 KV at a frequency of about 2-50 KHz,
preferably about 2-30 KHz. The upper portion of the corona
discharged treated electrocoated polymer layer is hydrophilic while
the underlying bulk of the polymer layer remains oleophilic. During
laser ablation of the polymer layer, the ablation process may be
controlled so that the upper portion of the polymer layer is
ablated but the underlying metal substrate is not exposed. In this
manner, portions of the polymer layer are hydrophilic (where not
ablated) and other portions are oleophilic (where the corona
discharge treated polymer has been ablated.)
When the laser-ablatable electrocoated polymer includes inorganic
particles, the particles may include metal oxides, preferably
aluminum oxides. The inorganic particles may be co-deposited with
the electrocoated polymer at approximately 5 wt. % or the inorganic
particles may be applied to the surface of the electrocoated
polymer layer prior to curing thereof.
In a third embodiment of the invention, the printing plate further
includes a hydrophilic second layer or overlayer on top of the
electrocoated polymer layer. More than one hydrophilic overlayer
may be included in the sheet, however, the present invention is
described hereinafter with regard to a single hydrophilic
overlayer. This is not meant to be limiting in that the present
invention includes the use of one or more hydrophilic overlayers.
The hydrophilic overlayer may have the same or different affinity
for printing fluid as does the electrocoated polymer layer or the
underlying substrate or both. At least one of the electrocoated
polymer layer and the hydrophilic overlayer includes
laser-sensitive particles to render the layer containing those
particles (and any overlying layer) ablatable by a laser.
The hydrophilic overlayer may include a) a hydrophilic polymer, b)
a hydrophilic polymer composition containing dye or inorganic
particles, c) a silicone polymer or copolymer composition
containing inorganic particles in a concentration sufficient to
make the silicone composition hydrophilic or d) a solvent borne
composition containing dye or inorganic particles.
A preferred hydrophilic polymer is an organophosphorus compound. As
used herein, the term "organophosphorus compound" includes
organophosphoric acids, organophosphonic acids, organophosphinic
acids, as well as various salts, esters, partial salts, and partial
esters thereof. The organophosphorus compound may be copolymerized
with acrylic acid or methacrylic acid. Copolymers of vinyl
phosphonic acid are preferred, especially copolymers containing
about 5-50 mole % vinyl phosphonic acid and about 50-95 mole %
acrylic acid and having a molecular weight of about 20,000-100,000.
Copolymers containing about 70 mole % acrylic acid groups and about
30% vinylphosphonic acid groups are particularly preferred. The
hydrophilic polymer may be applied in batch processing of sheet or
in coil processing by conventional coating processes including roll
coating, powder coating, spray coating, vacuum coating, immersion
coating or anodic electrodeposition. Preferably, the hydrophilic
polymer is applied by roll coating, typically to a thickness of
about 0.01-1.0 mil, preferably about 0.1-0.3 mil.
The dye preferably includes an azine compound or an azide compound
or any other dye that absorbs light in the range of about 500 to
about 1100 nanometers. A preferred dye is Nigrosine Base BA
available from Bayer Corporation of Pittsburgh, Pa. The inorganic
particles may be particles of a metal, mineral or carbon as
described above, and preferably are oxides of transition metals.
Particularly preferred inorganic particles include manganese oxide,
magnesium oxide and iron oxide. The dye or inorganic particles may
be solvated or suspended in an organic solvent such as methyl ethyl
ketone or nigrosine. The solution is applied to the electrocoated
polymer layer by roll coating or spray coating, and the solvent is
removed leaving a hydrophilic overlayer of the dye or inorganic
particles. When the overlayer includes a vinyl phosphonic acid
copolymer and an azine dye, a preferred concentration of the dye is
about 1-10 wt. %, preferably about 3-5 wt. %. When the overlayer
includes a vinyl phosphonic acid copolymer and manganese oxide, a
preferred concentration of manganese oxide particles having an
average particle size of about 0.6 micron is about 1-15 wt. %.
When the dye is applied as the overlayer alone or in combination
with a hydrophilic polymer, the underlying electrocoated polymer
may be uncured or cured. The electrocoated polymer may be cured
before the overlayer is applied or after the overlayer is
applied.
The overlayer may include a silicone polymer or copolymer
composition containing inorganic particles in a concentration
sufficient to make the silicone composition hydrophilic. Silicone
polymers or copolymers are typically hydrophobic and oleophobic.
However, when inorganic particles are included in a composition of
a silicone polymer or copolymer at a sufficient concentration, the
composition is hydrophilic and may be used as the hydrophilic
overlayer. Suitable silicone compositions include fluorosilicone,
dimethyl silicone, diphenyl silicone, and nitryl silicone. The
silicone composition may include additional particles such as
carbon black, graphite, silica, iron oxide, zinc oxide, zirconium
silicate, metal powders, and clays at a concentration of about
0.5-38 wt. %.
When the overlayer contains laser-sensitive particles (e.g. dye or
inorganic particles), the overlayer may be laser-ablated in an
image area to expose the underlying oleophilic electrocoated
polymer layer leaving a background area of the non-ablated
hydrophilic overlayer. The underlying electrocoated polymer layer
may include laser-sensitive particles and also be laser-ablatable.
Following laser-ablation of at least the overlayer, ink will adhere
to the image area while the background area will be covered with
water or a fountain solution. Alternatively, the background area
may be laser-ablated to render the background area oleophilic and
retain the image area as hydrophilic so that ink adheres to the
background area and water or fountain solution adheres to the image
area.
When the overlayer does not include laser-sensitive particles, the
underlying electrocoated polymer layer includes laser-sensitive
particles to render the electrocoated polymer layer
laser-ablatable. In this case, the electrocoated polymer layer is
ablated during laser imaging such that the hydrophilic overlayer
and at least a portion of the electrocoated polymer layer are
removed creating a hydrophilic area of unremoved overlayer and an
oleophilic area of unremoved electrocoated polymer. Alternatively,
the electrocoated polymer layer may be fully ablated to expose the
underlying substrate creating a hydrophilic area of unremoved
overlayer and an oleophilic area of the exposed substrate.
In a fourth embodiment of the invention wherein a lithographic
plate is desired for use with waterless printing solutions, the
printing plate includes an overlayer formed from a silicone polymer
or silicone copolymer, collectively referred to hereinafter as a
silicone overlayer. The silicone overlayer is preferably applied by
roll coating, typically to a thickness of about 0.01-1.0 mil,
preferably about 0.1-0.3 mil. The silicone overlayer is both
hydrophobic (repels water) and oleophobic (repels ink). In use, the
silicone overlayer is laser-ablated in the image area or in the
background to expose the underlying oleophilic electrocoated layer.
Ink of a waterless printing solution will adhere to the exposed
region of the electrocoated layer and will be repelled by the
non-ablated region.
BRIEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the invention will be obtained from the
following description when taken in connection with the
accompanying drawing figures wherein like reference characters
identify like parts throughout.
FIG. 1 is a schematic, top plan view of a first embodiment of the
lithographic printing plate made in accordance with the present
invention after exposure to the laser beams shown in FIG. 2;
FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG.
1;
FIG. 3 is a schematic, top plan view of a second embodiment of the
lithographic printing plate of the present invention after exposure
to the laser beam shown in FIG. 4;
FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG.
3;
FIG. 5 is a schematic, top plan view of third and fourth
embodiments of the lithographic printing plate of the present
invention after exposure to the laser beam shown in FIG. 6; and
FIG. 6 is a cross-sectional view taken along the line 5--5 of FIG.
5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For purposes of the description hereinafter, the terms "upper",
"lower", "right", "left", "vertical", "horizontal", "top", "bottom"
and derivatives thereof relate to the invention as it is oriented
in the drawing figures. However, it is to be understood that the
invention may assume various alternative variations and step
sequences, except where expressly specified to the contrary. It is
also to be understood that the specific devices and processes
illustrated in the attached drawings, and described in the
following specification, are simply exemplary embodiments of the
invention. Hence, specific dimensions and other physical
characteristics related to the embodiments disclosed herein are not
to be considered as limiting.
In FIGS. 1 and 2 there is shown the first embodiment of printing
plate 11 made in accordance with the present invention. The
printing plate 11 includes an unanodized aluminum alloy substrate
12 having a principal surface 13 coated with a laser-ablatable
layer 15. The substrate 12 has a thickness of about 8.8 mils. The
laser-ablatable layer 15 has a thickness of about 0.1 mil (2.5
microns) and contains about 95 wt. % of a mixture of acrylic and
epoxy polymers, together with about 5 wt. % iron oxide particles
having an average particle size of less than about 1 micron. The
layer 15 is applied to the sheet surface 13 by electrocoating.
Laser beams 20, 21 shown in FIG. 2 impinge upon the laser-ablatable
layer 15 and removes the layer 15 in the area corresponding to the
background of the image, thereby producing the image area 25 shown
in FIG. 1. The image area 25 is wettable by oleophilic printing
inks and the principal surface 13 of FIG. 1 is water-wettable
(hydrophilic).
FIGS. 3 and 4 show printing plate 11a of the second embodiment of
the present invention. The sheet 11a includes the layer 15, and an
upper portion 15a of the layer 15 which is hydrophilic. When the
upper portion 15a is ablated by the laser beam 20 as shown in FIG.
4, the underlying layer 15 is exposed creating an image area 25a
(FIG. 3) which is oleophilic. During laser-ablation of the layer
15a, some of the layer 15 may be ablated as well or the ablation
may be controlled to remove only the upper portion 15a and none of
the layer 15.
FIGS. 5 and 6 show printing plate 31 of the third and fourth
embodiments of the present invention. In the third embodiment,
printing plate material 31 includes an unanodized aluminum alloy
sheet substrate 32 having a principal surface 33 coated with a
polymer layer 35. The substrate 32 has a thickness of about 8.8
mils. The polymer layer 35 has a thickness of about 0.1 mil (2.5
microns) and contains about 95 wt. % of a mixture of acrylic and
epoxy polymers, together with about 5 wt. % iron oxide particles
having an average particle size of less than about 1 micron. The
polymer layer 35 is applied to the principal surface 33 by
electrocoating. The polymer layer 35 is overcoated with an
overlayer 36 having a thickness of about 0.01-0.3 mil. The
overlayer 36 preferably comprises a hydrophilic water-wettable
copolymer of acrylic acid and vinylphosphonic acid containing about
70 mole % acrylic acid groups and about 30 mole % vinylphosphonic
acid groups. The copolymer has an average molecular weight of about
50,000 to 80,000. The overlayer 36 may contain additives of a dye
or particles of carbon, metals or minerals or combinations thereof
as described above.
As shown in FIG. 6, when laser beam 20 impinges upon the overlayer
36 and removes the overlayer 36 in the area corresponding to the
image, an image area 45 is produced as shown in FIG. 5. The image
area 45 is wettable by oleophilic printing inks and the background
area of the overlayer 36 is hydrophilic. During laser-ablation of
the overlayer 36, some of the layer 35 may be ablated as well or
the ablation may be controlled to remove only the overlayer 36 and
none of the layer 35.
Alternatively, the overlayer 36 may be formed from a silicone
polymer or silicone copolymer and have a thickness of about
0.01-0.3 mil. The silicone overlayer is non-wettable by water
(hydrophobic) and non-wettable by oleophilic printing inks
(oleophobic). In this alternative embodiment, the sheet material 31
is useful for waterless printing processes. Upon laser-ablation of
the silicone overlayer, the resulting image area 45 is oleophilic
while the remaining background area is hydrophobic and oleophobic.
This printing plate material is useful for printing with a
waterless printing solution which will adhere to the image area 45.
When a fountain solution is desired for printing, the background
area can be modified to be hydrophilic by including additives of a
dye or particles of carbon, metals, or minerals as disclosed above
and combinations thereof in sufficient quantities. In that case,
the additive-modified silicone overlayer 36 is hydrophilic and the
image area 45 is oleophilic.
Although the invention has been described generally above, the
particular examples give additional illustration of the product and
process steps typical of the present invention.
EXAMPLE
Printing plate material was prepared according to the present
invention by roll texturing a front side of a test sheet (Sheet A)
of an Aluminum Association 3000 series alloy with an electron
discharge textured (EDT) roll to create a diffuse surface. Sheet A
was electrocoated with a layer 0.1 mils thick of about 95 wt. % of
a mixture of acrylic and epoxy polymers, together with about 5 wt.
% iron oxide particles having an average particle size of less than
about 1 micron. A control sheet (Sheet B) of an Aluminum
Association 3000 series alloy was mill finished (rolled with
standard mill rolls and no EDT) and was electrocoated as for Sheet
A. The front side and backside of Sheet A and the front side of
Sheet B were tested at several positions for total reflectance and
diffuse reflectance using a Milton Roy spectrophotometer at 550 nm
and the specular reflection was calculated as the difference
between the total reflectance and the diffuse reflectance as set
forth in Table 1. Tests were run at two longitudinal positions
along the sheet (Locations 1 and 2) with readings taken at the
edges (locations a and b) and the center of the sheet (location
c).
TABLE 1 Total Diffuse Specular Sheet Location Reflectance
Reflectance Reflectance A-front 1-a 76.0 57.2 18.8 A-front 1-b 75.8
56.1 19.7 A-front 1-c 78.8 62.8 16.0 A-front 2-a 75.7 58.0 17.7
A-front 2-b 77.7 58.5 19.2 A-front 2-c 78.1 62.1 16.0 A-back 1-a
74.1 47.7 26.4 A-back 1-b 74.7 45.4 29.3 A-back 1-c 77.9 41.4 36.5
A-back 2-a 73.6 47.5 26.1 A-back 2-b 76.4 45.6 30.8 A-back 2-c 78.4
40.2 38.2 B 1-a 74.4 49.1 25.3 B 1-b 74.3 47.7 26.6 B 1-c 74.5 47.8
26.7 B 2-a 74.3 48.3 26.0 B 2-b 74.6 47.5 27.1 B 2-c 74.3 48.9
25.4
The front side of Sheet A demonstrated significantly more diffuse
reflection than the backside of Sheet A and than the control of
Sheet B. The uniform roughness created by roll texturing of the
front side of Sheet A minimizes specular reflectance and increases
the uniformity of the sheet and the impact of an ablating laser
thereon in the longitudinal and transverse directions.
Having described the presently preferred embodiments, it is to be
understood that the invention may be otherwise embodied within the
spirit and scope of the appended claims.
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